This invention relates to polyphase full-wave rectifying circuits used between a polyphase AC line having a neutral point and a DC line, and more particularly to a circuit configuration which allows the absorption of surges developed on the AC or DC side and is preferably applicable, for example, to rectifying circuits between three-phase AC generators and regulators for automobile.
FIGS. 1A to 1D are examples of conventional three-phase full-wave rectifying circuits set between an AC generator (Y-connection three-phase synchronous generator) and a regulator of an automobile.
FIG. 1A is a basic bridge-type three-phase full-wave rectifying circuit using ordinary diodes D1-D6 as rectifying elements for each one of the 6 arms. The configuration incorporating 6 rectifying elements in one rectifying circuit is generally referred to as the six-in-one type. Another configuration that has two more arms provided with diodes D7 and D8 designated by dashed lines, and an interconnection point which is connected to the neutral point of an AC line is also well known. The configuration equipped with such rectifying elements for the neutral point is generally called the eight-in-one type. The eight-in-one type has the advantage of generating 10% more power than the six-in-one type.
This basic bridge-type three-phase full-wave rectifying circuit shown in FIG. lA has a disadvantage of an inability to absorb surges developing both inside and outside of the generator. Surges are classified into load damping surges (called internal surge) developing inside the generator at the time when a load and a capacitor (Battery) are disconnected, and other surges such as ignition surges developing outside the generator (and called external surges). In the circuit shown in FIG. 1A, neither internal surges nor external surges can be absorbed and some surges may break rectifying elements and other electronic circuits.
FIG. 1B is a first example of conventional circuit configurations having a larger surge absorptivity. In this circuit, Zener diodes ZD1-ZD6 (ZD7 and ZD8 can be added) are used as rectifying elements for all arms, allowing internal and external surges to be absorbed. This circuit, however, has disadvantage in that it requires a number of expensive Zener diodes.
FIG. 1C is a second example of a conventional circuit configuration which can absorb surges. All the diodes used on the positive output terminal T(+) side are ordinary diodes D1, D3 and D5 (with D7 optional), and only on the negative output terminal side are T(-) Zener diodes ZD2, ZD4, and ZD6 (with ZD8 optional) used. Although this configuration can decrease the number of Zener diodes, external surges remain without being absorbed in return.
FIG. 1D is a third example of a conventional circuit configuration which decreases the number of Zener diodes and still can absorb external surges. In this configuration all the diodes on the negative output terminal T(-) side, ZD2, ZD4 and ZD6 (with ZD8 optional), and one of the diodes on the positive output terminal side T(+) ZD1 are Zener diodes. This configuration can absorb both the internal and external surges, but still has a considerable number of Zener diodes.
As described above the conventional polyphase full-wave rectifying circuit which can absorb internal and external surges still involve problems of having to use a considerable number of Zener diodes.
It is another problem that the surge clamping voltage level for the internal surges (measured between the output terminals) differs from that for external surges. As defined in FIGs. 1B and 1D, internal surges are clamped by one Zener diode ZD2, ZD4 or ZD6 at each phase, while external surges are clamped by two Zener diodes ZD1 and ZD2 connected in series. Therefore the clamping voltage for internal surges results in the same level as that of the Zener voltage, and that for external surges becomes twice as high as that of the Zener voltage. This difference in the clamping voltage level makes these conventional circuits very difficult to use.
Furthermore, in the eight-in-one type, for ease of assembly, rectifying elements generally used for the neutral point have the same capacity as that of other rectifying elements. However in practice, since the current flowing into the rectifying elements for the neutral point is only about one tenth the strength of that flowing into other rectifying elements, it can be said that the elements for the neutral point are not being effectively used.